The present invention relates to a plasma processing system
Plasma processing systems of this kind, which use inductively coupled plasma sources, are suited, in particular, for deeply etching silicon at very high etching rates, employing the method described in German Patent DE 42 41 045, and are widely known. A simple and proven arrangement is made up of an ICP coil (ICP=xe2x80x9cinductively coupled plasmaxe2x80x9d), which is wound around a plasma volume and is fed by a high-frequency a.c. voltage. In the plasma volume, the high-frequency currents flowing through the ICP coil induce a high-frequency, magnetic alternating field, whose electric curl (rotational or vortex) field, in turn, excites the plasma in accordance with the law of induction (rotE=xe2x88x92∂B/∂t). The applied high frequency has values of between 600 kHz and 27 MHz; a frequency of 13.56 MHz is usually used.
In the method known from German Patent DE 42 41 045, a plasma source, preferably having inductive high-frequency excitation, is used to liberate fluorine radicals from a fluorine-supplying etching gas, and passivation gas (CF2)x from a Teflon-forming monomer, the plasma source generating a highly dense plasma having a relatively high density of ions (1010-1012 cmxe2x88x923) of a low energy, and the etching and passivation gases being alternately used. The ionic energy, which accelerates the generated ions toward the substrate surface, is likewise relatively low, i.e., between 1-50 eV, preferably 5-30 eV. In the description, FIG. 2 of this patent shows a typical asymmetrical supplying of an ICP coil of such a plasma source, as known from the related art. In the simplest case, it is composed of only one single winding around a reactor in the form of a vessel (tank or chamber) made of ceramic material, having a diameter of, for example, 40 cm. One coil end is grounded; the other coil end is fed with the high-frequency a.c. voltage, and described as xe2x80x9chotxe2x80x9d, because at this coil end, very high voltages of, for example, 1000-3000 volts build up, which are typical for the amplitude of the supplied high-frequency high voltage.
The capacitors C2 and C3, likewise shown in FIG. 2, are used to adapt the impedance of an asymmetrical 50 xcexa9 coaxial cable output of a high-frequency incoming supply to the impedance of the asymmetrically operated ICP coil (so-called xe2x80x9cmatchboxxe2x80x9d or xe2x80x9cmatching capacitorsxe2x80x9d). The capacitor C4 is connected in parallel to the ICP coil and, together with the matching capacitors, produces the resonance condition.
The consequence of supplying the plasma source in the known asymmetrical and inductive fashion is that the asymmetry is also projected onto the plasma that is produced. On the average, depending on the intensity of an occurring capacitive coupling, this lies above earth potential by a few up to tens of volts. Thus, one coil end of the ICP coil is at earth potential (0 V) and the opposite xe2x80x9chotxe2x80x9d coil end is at a high high-frequency voltage of up to a few thousand volts. As a result, strong electrical fields are induced in the plasma, in particular at the xe2x80x9chotxe2x80x9d coil end, through the ceramic vessel wall of the reactor, which, in turn, produce displacement currents in the plasma, through the ceramic vessel wall. In this case, one speaks of the already mentioned xe2x80x9ccapacitive couplingxe2x80x9d, while the actual production of plasma is an inductive mechanism, i.e., one based on time-variable magnetic fields.
For the most part, the capacitive coupling causes a current to flow from the supplied, i.e., xe2x80x9chotxe2x80x9d coil end, through the ceramic vessel wall of the reactor, into the plasma. Since the average plasma potential fluctuates near earth potential, to which the xe2x80x9ccoldxe2x80x9d coil end is also fixed, and since the potential difference between the plasma and the xe2x80x9ccoldxe2x80x9d coil end is too small to allow the displacement current to flow off (discharge) again across the ceramic vessel wall to the coil, this current flow is not able to flow off (discharge) to the grounded coil end. Thus, the displacement current must discharge (flow off) from the region of the xe2x80x9chotxe2x80x9d coil end into the plasma, again out of the plasma, across a ground that is in direct contact with the plasma. In known methods heretofore, this is essentially the substrate electrode, which, for example, as a substrate, bears a wafer, and which is operated via a separate high-frequency supplying at a low negative DC bias potential of 1-50 V with respect to the plasma. Therefore, it is able to directly take up the mentioned displacement currents. However, this leads to inhomogeneities in the plasma-processing method in question, across the substrate surface and, thus, partially to considerable profile deviations when etching in individual regions.
In addition, the strong electrical fields occurring on one side, due to the asymmetrical supplying, distort the position and density distribution of the produced plasma, which is shifted away from the middle of the reactor and is displaced, for example, toward the xe2x80x9chotxe2x80x9d coil end. In this case, one speaks of a so-called xe2x80x9cbull""s eye shiftxe2x80x9d, since the inhomogeneity of the plasma is projected in the shape of an eye onto the wafer used as a substrate, and since this xe2x80x9ceyexe2x80x9d shifts away from the middle of the wafer toward the wafer rim.
A first measure for improving the process homogeneity and for avoiding the xe2x80x9cbull""s eye shiftxe2x80x9d is a process known from German Patent DE 42 41 045, as described in the unpublished Application DE 197 34 278.7. An aperture construction is proposed, which, due to an expanded ionic recombination zone at the inner wall of a metal cylinder mounted on an aperture, homogenizes the ionic flow toward the substrate across the substrate surface in question. It does this by incorporating an ionic loss mechanism in the outer region of the plasma attaining the substrate. It also recenters the plasma and partially shields electrical fields coming from the source region of the plasma, on the way to the substrate.
A further measure, which diminishes profile deviations of etched structures on the substrate or a wafer, occurring, in part, as the result of electrical interference fields, is proposed in the unpublished Application DE 197 363 70.9, in which a so-called xe2x80x9cparameter rampingxe2x80x9d is used.
In the same way as the xe2x80x9chotxe2x80x9d coil end, i.e., the xe2x80x9ccoldxe2x80x9d coil end, the grounded coil end in the related art, is a problem zone, because this end is the location of minimal feeding in (injection) or feeding out (coupling out) of displacement current, due to the capacitive coupling into the produced plasma. Moreover, in known methods heretofore, the corresponding xe2x80x9ccoldxe2x80x9d voltage feed point of the ICP coil, which is in communication with the xe2x80x9ccoldxe2x80x9d coil end, must be grounded with great care, since, in the active ambient environment of the plasma reactor, vertically flowing currents, in particular, must be avoided at all costs, i.e., currents flowing from the ICP coil down to the grounded housing. Such vertical currents, in other words currents that do not flow in parallel to the coil plane defined by the ICP coil, result in a time-variable magnetic field that is tilted by 90xc2x0 and that has corresponding electrical induction effects over the electric curl (rotational or vortex) field. This leads to considerable local disturbances in the plasma, which are manifested, in turn, in profile deviations (pocket formations, negative etching-edge slopes, mask rim undercuts).
Another interference effect that is known in related-art methods and is caused by high voltages at a xe2x80x9chotxe2x80x9d coil end, entails sputtering deposition at this end, on the inside of the reactor side wall, by ion bombardment, i.e., by positively charged ions accelerated by strong electrical fields toward the chamber wall. In the process, sputtered wall material can also attain the wafer or the substrate and have a micro-masking effect there. The consequence, as is generally known, is the formation of silicon needles, micro-roughness, or silicon particles. Since the sputter-type ablation of the reactor side wall scales the applied high-frequency voltage, to achieve minimal sputter rates, it is desirable that the high-frequency voltage applied to the ICP coil be kept to a minimum toward the plasma.
One approach, already known from the related art, proposes using a transformer to symmetrically feed the ICP coil. The transformer is supplied on the primary side with a high-frequency a.c. voltage via a high-frequency feed-in, and has a secondary coil with a grounded, middle tapping point. This enables the transformer to supply both ends of the ICP coil of a plasma-processing system with a high-frequency, 180xc2x0 out-of-phase high voltage of at least nearly the same amplitude. A transformer of this kind is usually constructed using a coil winding of litz wire on a ferritic core material, a cup-type or toroidal core being used as the ferritic core. A serious drawback that transformers of this kind have, however, is that high magnetic losses, ranging from 10-20%, occur in the core materials at frequencies of, for example, 13.56 MHz. This leads to considerable thermal problems in the case of high high-frequency power of usually between 500 to 3000 watts, in ICP plasma-processing systems. Moreover, phase errors also occur due to the frequency-dependent energy absorption through the core material in the transformer. At the symmetrical output, these phase errors, in part, substantially distort the 180xc2x0 phase of the two high voltages that need to be injected in phase opposition. In addition, transformers of this kind have the undesirable effect of limiting the amount of high-frequency power that can be injected into the plasma.
The plasma processing system in accordance with the present invention having the characteristic features of the main claim has the advantage over the related art that both coil ends are supplied via a symmetrical coil in-feed with a high-frequency voltage of the same frequency, the ICP coil being symmetrically supplied with two high-frequency a.c. voltages, in phase opposition to one another, at a first coil end and a second coil end by a xcex/2 delay line (slow-wave structure) (a so-called cable balun (balanced to unbalanced transformer)) provided between and connecting a first voltage feed point and a second voltage feed point. Independently of voltage and power, the xcex/2 delay line effects a 180xc2x0 phase shift of the voltage U(t) supplied at a first voltage feed point and, thus, an in-feed of xe2x88x92U(t) at a second voltage feed point. Thus, without entailing substantial technical outlay or additional cost-intensive components, two high-frequency, 180xc2x0 out-of-phase a.c. voltages having the same frequency and at least nearly the same amplitude are produced from one voltage made available by the high-frequency supplying. This renders possible especially simple, low-loss plasma processing or plasma etching methods that are suitable for high high-frequency power (several kilowatts), for inductive plasma sources, in particular.
In addition, in the case of the symmetrical supplying of the ICP coil in accordance with the present invention, both coil ends become xe2x80x9chotxe2x80x9d, in other words both coil ends now conduct a high-frequency a.c. voltage of considerable, in the ideal case, of identical amplitude, which occurs at both coil ends in exact phase opposition. Thus, if a voltage Û(t) is applied at the first coil end, then a voltage xe2x88x92Û(t), whose amplitude is only still half as great as the asymmetrical supplying in the related art, is applied accordingly to the second coil end, because the original a.c. voltage 2*Û(t) to ground is now divided into Û(t) and xe2x88x92Û(t) to ground. By halving the voltage amplitude at the two coil ends in this manner, disturbing wall sputter rates at the inner reactor wall are initially drastically reduced.
The concentration of undesirable high-energy ions is also reduced very advantageously. These high-energy ions are otherwise accelerated by high electrical fields toward the reactor wall, are reflected there, and, consequently, returned to the produced plasma. There, they affect the processed substrate in numerous disturbing ways, such as by causing profile disturbances, or damage to the oxide layers on an etched wafer. At the same time, the concentration of high-energy electrons in the produced plasma also decreases, since the capacitive current injection into the produced plasma is substantially reduced by the symmetrical coil feeding (at least by a factor of 2), so that the electron gas is no longer heated in a relevant manner. The beneficial result is that the produced plasma becomes colder. Besides this, high-energy electrons are generally undesirable for the plasma processes, because they absorb high-frequency power unnecessarily.
Since the electrical stray fields from the coil region are inversely the same in the case of symmetrical coil feeding, they also even out very advantageously, so that the so-called xe2x80x9cbull""s eye shiftxe2x80x9d no longer occurs.
In addition, the currents, which are capacitively injected into the produced plasma and which, as mentioned, are already perceptibly smaller, are now likewise inversely the same, i.e., they even out (become equalized) between the coil ends and, very advantageously, no longer flow via grounding systems, such as the substrate electrode and the processed substrate, that are in direct contact with the plasma.
Since in the plasma-processing system of the present invention, at every instant, each of the two coil ends of the ICP coil receives the negative voltage value of the other associated coil end, the displacement current induced in the produced plasma by one coil end, via the ceramic reactor wall as a dielectric, can likewise be taken up by the other coil end, likewise via the ceramic reactor wall as a dielectric, so that there is no need for a charge compensation via a potential to ground, i.e., via the substrate surface. This substantially improves etching-rate and profile homogeneity across the substrate surface, and fewer profile deviations occur.
In addition, the plasma potential advantageously approaches is the ground potential, since electrical injections into the plasma clearly diminish, and the remaining injections are equalized because of their symmetry. In addition, the plasma symmetry also increases advantageously, since by removing the capacitive injection distorting the plasma or by equalizing the injection, one obtains a colder and, in the ideal case, a rotationally symmetric plasma, without any obvious potential differences at individual points in the plasma (plasma locations).
Advantageous embodiments of the present invention are derived from the measures delineated in the dependent claims.
Thus, it is also advantageous, even given a symmetrical supplying of the ICP coil, for all current-carrying conductors in an active ambient environment of the reactor or the ICP coil, to be run in parallel to the coil plane defined by the ICP coil. Ambient environment is understood, in this context, to be a region around the reactor and the ICP coil, in which relevant disturbing influences are caused by electromagnetic interactions between the currents flowing in the conductors and the produced plasma. Thus, the only currents flowing in the vicinity of the produced plasma, are concentrated, parallel-conducted currents, which do not induce any harmful magnetic interference fields in the plasma. As a result, the plasma has fewer disturbances and is colder, and the risk of the substrate being damaged by high-energy ions or electrons is considerably reduced. At the same time, the plasma potential is also decreased and approaches the ground potential.
By combining the xcex/2 delay line with a preferably symmetrical capacitive network installed between the two voltage feed points and the two coil ends in order to adapt impedance to the produced inductive plasma, one can advantageously achieve a symmetrical supplying of the ICP coil that is virtually loss-less. In combination with a reduction in the supply voltage amplitudes at the two coil ends, the symmetrical feeding of the ICP coil in accordance with the present invention thus permits very high feed-in power into the produced plasma, as an inductive plasma source, reaching into the range of several kilowatts, as well as a scaling-up of the performance parameters of the plasma-processing system, thereby ultimately leading to higher etching rates.
Since in the case of the symmetrical coil feeding in accordance with the present invention, both coil ends are xe2x80x9chotxe2x80x9d, even if at a lower level (lesser extent), it continues to be very beneficial to configure both xe2x80x9chotxe2x80x9d coil ends at an increased distance from the ceramic vessel, which surrounds the produced highly dense plasma in the form of a reactor. This is achieved most simply in that the ICP coil producing the plasma, which at least substantially surrounds the reactor vessel on the outside on a region by region basis, has a somewhat greater diameter than the outer reactor diameter and, thus, is placed around the reactor in such a way that the side of the ICP coil opposing the two coil ends just touches the ceramic of the reactor vessel. The reactor vessel circumference is thus tangent to the larger coil circumference surrounding it at the coil side disposed diametrically opposite from the coil ends. In this manner, the distance of the ICP coil to the plasma produced inside the reactor increases with rising electric potential, the two coil ends, as the locations of the highest electrical potential, having the maximum distance to the reactor. For this, approx. 1-2 cm already suffice. Here as well, it is quite significant that all current-carrying conductors in the ambient environment of the reactor run horizontally in the coil plane defined by the ICP coil, to keep disturbing high-frequency magnetic fields away from the plasma.
In addition, the symmetrical coil feeding can be advantageously combined with the aperture construction proposed in the unpublished Application DE 197 34 278.7. This further reduces the high-frequency magnetic field of the ICP coil at the location of the substrate and homogenizes the plasma density distribution.
In addition or alternatively to the mentioned aperture construction, in the plasma processing system according to the present invention, it is advantageously possible to place a circular, metallic spacer in the reactor side wall, between the plasma source and the substrate electrode, to reduce the influence of high-frequency magnetic fields from the region of the produced, highly dense plasma or the ICP coil, on the substrate, or, for example, on a silicon wafer arranged there. For this, the spacer preferably has a height of about 10 cm-30 cm and is preferably made of aluminum or of another metal that is resistant to the plasma process. Its use increases the distance between the plasma source, i.e., the location for producing the highly dense plasma through inductive coupling, and the substrate electrode which bears the substrate, and consequently lessens the influence of magnetic and electrical fields, which decrease as a function of the distance r, such as at least by 1/r.
Since the components of the plasma processing system in accordance with the present invention are not subject to any theoretical performance limitations, the system can thus be driven at very high source power in the kilowatt range in a virtually loss-less manner. Due to the lack of energy-absorbing components, the required phase relations between the coil connections are fully retained, independently of the supplied power, and no special measures are required for cooling components. Thus, the present invention provides an excellent reproducibility and reliability of operation for the plasma processing system. In particular, by halving the high-voltage amplitude at both coil ends, even more free space is created for even greater plasma capacity, in that the performance parameters are scaled up, so that very high etching rates are attained for the silicon.
At the same time, a multiplicity of disturbing effects is clearly diminished by the plasma-processing system according to the present invention. These include: wall sputtering, micro-masking caused by sputtered particles, the production of high-energy ions or of very hot electrons in the plasma, an undesirable energy dissipation, a capacitive injection of displacement currents, a distortion of the plasma caused by electrical fields, a shifting of the plasma distribution, an increase in and distortion of the plasma potential, inhomogeneities within the produced plasma, and equalizing currents, which flow across the substrate and substrate electrode to ground.